A solid electrolytic capacitor having a low ESR is used around a CPU of a personal computer. An aluminum electrolytic capacitor is used for smoothing in a power circuit, and the like. In the capacitors, there is a strong demand for miniaturization and having a large capacitance (capacity).
The conventional solid electrolytic capacitor includes an electrode foil (positive electrode foil) on which a dielectric film is formed, a solid electrolytic layer made of a conductive polymer formed on the dielectric film, and a negative electrode layer formed on the solid electrolytic layer. Recently, as illustrated in FIG. 21, electrode foil 3 including substrate 1 composed of a valve action metal foil and coarse film layer 2 including a void therein and formed on substrate 1 by evaporation is studied in order to increase the capacitance of the capacitor (for example, PTL 1).
Coarse film layer 2 is formed as an aggregation of tree-shaped or sea-grape-shaped columnar bodies 5 in each of which metallic microparticles 4 are stacked on the surface of substrate 1 and extend from the surface of substrate 1. As metallic microparticles 4 are stacked, a surface area of coarse film layer 2 is enlarged and the capacitance of the capacitor in which electrode foil 3 is used is increased.
In order to increase the capacitance of electrode foil 3, the number of stacking times of metallic microparticle 4 may be increased to enlarge the total surface area of coarse film layer 2. However, as the number of stacking times of metallic microparticle 4 is increased, coarse film layer 2 is thickened, which make it hard to miniaturize the capacitor. Sometimes strength of columnar body 5 against a stress load from a perpendicular direction is decreased when a height of columnar body 5 is increased.
When a particle diameter of metallic microparticle 4 is reduced, the total surface area of coarse film layer 2 can be enlarged. However, mechanical strength of coarse film layer 2 is decreased, and electrical insulation is easily generated in a connection portion between metallic microparticles 4, and therefore the reduced particle diameter of metallic microparticle 4 does not contribute to the increase in capacitance of electrode foil 3. As described above, it is difficult to increase the capacitance of electrode foil 3.